The alloying design idea,strengthening-toughening mechanism,microstructure,mechanical performances,development and application in China of new type Mn-series bainitic steels are introduced.Mn-series air-cooling bainitic steels including granular bainitic steels,FGBA /BG duplex steels,CFB/M duplex steels,medium carbon bainite/martensite steels,cast bainitic steels are presented.The invented idea mechanical performances,development and application of second generation of Mn-series bainitic steels,i.e.water quenching Mn-series bainitic steels invented by the authors newly are introduced.The water quenching Mn-series bainitic steels cover severe series steels containing ultra-low carbon,low-low carbon,medium-low carbon,and high-low carbon content etc,which can reduce the amount of alloying content,increase hardening capability and improve weldability.It should be pointed out that the application of both air cooling and water quenching Mn-series bainitic steels are complementary and mutually reinforcing,and the new type Mn-series bainitic steels can meet the performance requirements of most steels used in engineering structure.Some newest technologies of Mn-series bainitic steels in China are discussed in this paper.It is suggested that the significance of the development of the Mn-series bainitic steels can be summarized as:significantly reducing costs of both raw materials and production;good combination of strength and toughness;excellent weldability;simple procedure;large savings in energy resources and reduced environmental pollution. The alloying design idea, strengthening-toughening mechanism, microstructure, mechanical performances, development and application in China of new type Mn-series bainitic steels are described. Mn-series air-cooling bainitic steels including granular bainitic steels, FGBA / BG duplex steels, CFB / M duplex steels, medium carbon bainite / martensite steels, cast bainitic steels are presented. The invented idea mechanical performances, development and application of second generation of Mn-series bainitic steels, iewater quenching Mn-series bainitic steels invented by the authors newly are introduced. The water quenching Mn-series bainitic steels cover severe series steels containing ultra-low carbon, low-low carbon, medium-low carbon, and high-low carbon content etc, which can reduce the amount of alloying content, increase hardening capability and improved weldability. It should be pointed out that the application of both air cooling and water quenching Mn-series bainitic steels are complementary and mutuall y reinforcing, and the new type Mn-series bainitic steels can meet the performance requirements of most steels used in engineering structure. Some newest technologies of Mn-series bainitic steels in China are discussed in this paper. It is suggested that the significance of the development of the Mn-series bainitic steels can be summarized as: significantly reducing costs of both raw materials and production; good combination of strength and toughness; excellent weldability; simple procedure; large savings in energy resources and reduced environmental pollution.